Mechanism of ethanol/water reverse separation through a functional graphene membrane: a molecular simulation investigation

doi:10.1007/s11705-022-2246-z

Front. Chem. Sci. Eng.

2023, Vol. 17

Issue (3) : 347-357 https://doi.org/10.1007/s11705-022-2246-z

RESEARCH ARTICLE

Mechanism of ethanol/water reverse separation through a functional graphene membrane: a molecular simulation investigation

Quan Liu^1,², Xian Wang¹, Yanan Guo²(

), Gongping Liu²(

), Kai-Ge Zhou³

¹. School of Chemical Engineering, Analytical and Testing Center, Anhui University of Science and Technology, Huainan 232001, China
². State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, China
³. Haihe Laboratory of Sustainable Chemical Transformations, Tianjin 300192, China

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Abstract

Reverse-selective membranes have attracted considerable interest for bioethanol production. However, to date, the reverse-separation performance of ethanol/water is poor and the separation mechanism is unclear. Graphene-based membranes with tunable apertures and functional groups have shown substantial potential for use in molecular separation. Using molecular dynamics simulations, for the first time, we reveal two-way selectivity in ethanol/water separation through functional graphene membranes. Pristine graphene (PG) exhibits reverse-selective behavior with higher ethanol fluxes than water, resulting from the preferential adsorption for ethanol. Color flow mappings show that this ethanol-permselective process is initiated by the presence of ethanol-enriched and water-barren pores; this has not been reported in previous studies. In contrast, water molecules are preferred for hydroxylated graphene membranes because of the synergistic effects of molecular sieving and functional-group attraction. A simulation of the operando condition shows that the PG membrane with an aperture size of 3.8 Å achieves good separation performance, with an ethanol/water separation factor of 34 and a flux value of 69.3 kg∙m^‒2∙h^‒1∙bar^‒1. This study provides new insights into the reverse-selective mechanism of porous graphene membranes and a new avenue for efficient biofuel production.

Keywords reverse separation graphene membrane ethanol/water separation molecular simulation

Corresponding Author(s): Yanan Guo,Gongping Liu

Online First Date: 15 December 2022 Issue Date: 17 March 2023

Cite this article:

Quan Liu,Xian Wang,Yanan Guo, et al. Mechanism of ethanol/water reverse separation through a functional graphene membrane: a molecular simulation investigation[J]. Front. Chem. Sci. Eng., 2023, 17(3): 347-357.

URL:

https://academic.hep.com.cn/fcse/EN/10.1007/s11705-022-2246-z
https://academic.hep.com.cn/fcse/EN/Y2023/V17/I3/347

Fig.1 Atomic structures of functional graphene membranes: (a–e) PG_D2.8, 3.0, 3.3, 3.6, 3.8 ?, respectively; (f–j) PG–OH_D2.8, 3.0, 3.3, 3.6, 3.8 ?, respectively (atoms: C, green; H, white; O, red).

Fig.2 Simulation system: equimolar ethanol/water separations in PV process.

Fig.3 Reverse-separation behavior in functional graphene: (a–b) the number of transferred ethanol and water molecules via the PG and PG–OH membrane when the pore size equals 3.6 ?, and (c) the molar flux is dependent on the pore size for PG and PG–OH membranes.

Fig.4 The visualized separation processes—snapshots of equimolar ethanol/water permeating through membranes at the equilibrium state: (a) PG_D3.6 ?, (b) PG–OH_D3.6 ? (water and ethanol molecules are represented by green and orange balls, respectively); number distributions along the z-direction at the last 40 ns of water and ethanol through membranes: (c) PG_D3.6 ?, (d) PG–OH_D3.6 ?. The membrane is located at the dotted line.

Fig.5 RDF for water and ethanol surrounding PG and PG–OH membranes: (a) D = 2.8 ?; (b) D = 3.6 ? (the influence of oxidized degree is compared in each figure); the specific atoms affect the affinity to penetrants: g(r) for (c) water and (d) ethanol around the carbon, oxygen and hydrogen atoms in membranes.

Fig.6 Density contours of water and ethanol distributed on PG and PG–OH membranes with a 3.6 ? size pore. For PG and PG–OH, the density contours of water and ethanol are compared in (a)–(b), and (c)–(d), respectively. The color bar represents the density contours, N/uc, equaling the number of molecules per unit grid with the density unit of 1/(1.25?³).

Fig.7 Density contours of water and ethanol distributed on PG and PG–OH membranes with a 2.8 ? size pore. For PG and PG–OH, the density contours of water and ethanol are compared in (a)–(b), and (c)–(d), respectively. The color bar and density unit are the same as in Fig. 6.

Fig.8 The MSD of water and ethanol diffusion through PG and PG–OH membranes: (a) PG_D2.8 ?, (b) PG–OH_D2.8 ?, (c) PG_ D3.6 ?, and (d) PG–OH_D3.6 ?.

Fig.9 Separation through a multilayered PG_D3.6 ? membrane: relationships between membrane flux and (a) the number of layers and (b) membrane thickness.

Fig.10 Simulation at operando conditions: (a) the simulation configuration of 10 wt % ethanol/water mixture permeating through the PG_D3.8 ? membrane, and (b) the effect of pore diameters on the separation performance (the black line denotes the separation factor of ethanol).

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